1. Field of the Invention
This invention relates to a method for producing plastic package base caps for sealing semi-conductor elements or semi-conductor integrated circuit units therein with respect to an overlying top cap.
2. Description of the Prior Art
One prior art method of sealing semi-conductor elements from the surrounding atmosphere is to place the semi-conductor element in a plastic material and by molding the plastic about the semi-conductor element, producing a directly embedded semi-conductor arrangement. In such system, however, the semi-conductor element is subjected to substantial stresses and strains due to temperature variations during the molding process or during use of the semi-conductor circuit.
This problem arises by reason of different coefficients of thermal expansion of the plastic material comprising the package and the semi-conductor element and will often lead to a break in the lead wires or complete destruction of the semi-conductor unit.
Recently, a cavity mold package arrangement has been developed wherein the semi-conductor element is not in direct contact with the plastic material of the package.
FIG. 1 illustrates a typical example of the semi-conductor unit of the cavity mold package arrangement. In this arrangement, a lead frame includes leads 1, 1' which extend through and are integrated with a plastic package 2 formed from a molded powdered epoxide polymer. A semi-conductor element 3 is mounted in the package 2 and is electrically connected to the lead frame by a conventional wire bonding technique. A top cap 5 is sealably secured to the package by using heat and pressure responsive adhesive, preferably epoxide resin adhesive 6.
With such system, however, it is difficult to place the lead frame in the powder plastic material during the molding of the package 2 in such a manner that the inner terminal ends of the leads 7 extending into the package cavity C are consistently coplanar with one another in the molded package.
When the inner terminal ends of the leads are not consistently flush, the bonding of the lead frame terminals to the fine internal lead wires used for connecting the lead frames to the semi-conductor element can not readily be carried out by using a wire bonder and this leads to the formation of bonds having poor bonding strength between the lead frame and the internal lead wires.
Under certain circumstances, the bonding operation itself becomes impossible.
Also, during the production of the packages of this type, it is necessary to provide a liquid honing step for removing the cured plastic material on the portions 7 of the lead frame which are disposed and uncovered within the hollow cavity after completion of the molding. The amount of cured plastic material built-up on the leads 7 is substantially large and the honing step consumes much time.
Further, because of contact of one side 8 of the lead frame terminal portions 7 located within the package cavity with the plastic material (bonded thereto during the molding operation), the gas-tight seal can easily be impaired or lost when the package is placed in an atmosphere with substantial temperature variations, due to the differential between the coefficients of thermal expansion of the cured plastic material and the lead frame.